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## 'Colour Islands Sudoku' printed from http://nrich.maths.org/

By Henry Kwok

#### Rules of Colour Islands Sudoku

As in a standard Sudoku, the object of the puzzle is to fill in the
whole 9x9 grid with numbers 1 through 9 so that each row, each
column, and each of the nine 3x3 squares must contain all the nine
different numbers 1 through 9.

There are two coloured "islands" in the 9x9 grid. Within each
"island" there are two extra rules to apply:

- no two diagonally adjacent cells can contain the same
number

- EVERY 3x3 square within the islands must contain the numbers 1
through 9.

For example the cell (5,3)
contains the number 5 so the cell (6,4) in the yellow island cannot
contain a 5 because it is diagonally adjacent to (6,4) - being to
the right and down one.

The 9x9 grid with corners at C4R3,
C4R5, C6R5 and C6R3 must contain all the digits 1 to 9.